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An atmospheric river storm that walloped the Bay Area on Thursday, causing traffic snarls, flood scares and at least one major mudslide that wrecked homes and cars, has finally left Northern California.
But apart from the hassles and damage, the biggest storm of the winter so far also delivered something quite valuable: a boost to the Sierra Nevada snowpack — the source of one-third of California’s water supply — to 102 percent of its historical average for April 1.
In other words, California already has the equivalent of an average winter’s snow supply, with six weeks still left to go in this year’s winter rain and snow season. Thursday’s snowpack was also 135 percent of the historical average for that date.
“We are happy where we are right now,” said Chris Orrock, a spokesman for the state Department of Water Resources. “It’s a good sign for our water supply. It’s looking like we are having an above-average year. It wasn’t that long ago that we had five years of historic drought.”
There’s more promising news on the horizon, forecasters say: Friday and Saturday’s weather should feature more rain, but much lighter, only about half an inch in most Bay Area cities.
“We’re going to keep having periodic showers off and on into Saturday,” said meteorologist Jan Null with Golden Gate Weather Services in Saratoga. “No significant amounts. You might have a 10-minute downpour, but that’s going to be about it.”
The extreme weather swings experienced by Californians the past seven years — a historic drought followed by drenching winter storms — will become the norm over coming generations, a study last year by researchers at UCLA and other institutions found. Those types of extremes are not new, but hotter global temperatures and warming oceans are putting more water vapor into the air, which can make atmospheric river storms stronger, even as hotter temperatures make summer droughts worse, concluded the study, which was published in the scientific journal Nature Climate Change.
Friday’s and Saturday’s storms are part of a cold front moving in from the Pacific Northwest. Temperatures are forecast to fall into the 40s in most Bay Area cities Friday morning, and the 30s at higher elevations.
With a snow level down to about 3,000 feet, that will mean new snow on some of the highest Bay Area peaks by mid-day, including Mount Hamilton, Mount Diablo and Mount Saint Helena in Napa County, said Scott Rowe, a forecaster with the National Weather Service in Monterey.
“Snow levels are going to start to drop rather rapidly,” Rowe said.
By Saturday night, the snow level should be down to 1,500 feet in the Bay Area. That would mean snow Sunday morning in the East Bay hills, across the Diablo Range, and throughout the Santa Cruz Mountains.
Forecast models show precipitation tapering off Sunday, with sunny conditions on Monday and Tuesday.
The break in the storms is critical. With lighter rain giving way to dry conditions, swollen creeks and rivers across Northern California will slowly recede, experts say.
“Most areas dodged a bullet,” Null said. “The rain is stopping just in time.”
Early Thursday morning, as the Guadalupe River in San Jose rose near flood level, San Jose city officials issued an evacuation alert for residents living near the river from Willow Glen Way to Atlanta Avenue. The alert went out at at 4:21 a.m. to cellphones, with an email going out at 4:37 a.m. saying residents were “encouraged to evacuate immediately due to threat of flooding.”
Several hundred people live in the area.Two hours later, after the city and the Red Cross opened an evacuation center, the city lifted the evacuation warning as the river began falling.
The river level crested at 8.7 feet at 5 a.m., just above its flood stage of 8.5 feet. By 4 p.m., it had fallen nearly two feet.
“It’s pretty much peaked. We’re not expecting flooding Friday,” said Colleen Valles, a spokeswoman for the Santa Clara Valley Water District.
Several other major rivers around the Bay Area either came within one foot of their flood stage or were expected late Thursday night to go one foot above it, including the Napa River and the Carmel River. The only major river where more serious flooding was anticipated was the Russian River at Guerneville, which was projected to crest at about 36 feet early Friday morning, roughly four feet above its flood stage of 32 feet.
Meanwhile, rainfall totals from the storm were substantial. Most Bay Area cities received between 10 to 15 percent of their average annual rainfall in two days.
For the 48 hours ending at 1 p.m. Thursday, San Francisco received 3.46 inches; Oakland, 3.09 inches and San Jose 1.33 inches.
Mountain areas were soaked with much more. Venado, in the Sonoma County hills, was drenched with a foot of rain — 12.64 inches — while Mining Ridge near Big Sur saw 9.77 inches; Mount Umunhum near Los Gatos 7.83 inches; Ben Lomond in the Santa Cruz Mountains 7.32 inches and Mount Diablo 4.25 inches.
The fiercest parts of the storm were felt in Sonoma and Marin counties, where Mount Tamalpais High School in Mill Valley was closed, and multiple schools in Sonoma County were closed. San Anselmo received 8 inches of rain during the storm, Mill Valley five and Santa Rosa five.
“If you are in the South Bay, you would might have thought, eh, I’ve felt bigger storms in the past 10 years,” Null said. “But if you are in the North Bay, this was one of the bigger storms in the past decade.”
The most serious incident came in Sausalito, where a duplex slid off its foundation in the 400 block of Sausalito Boulevard shortly after 3 a.m. Thursday. The mudslide sent the house down a steep hill, where it slammed into another home.
“One of our deputies was able to talk to a woman that was inside the home that slid off the hill,” the Marin County Sheriff’s Department said on its Facebook page. “The woman was trapped inside and could not get out. The deputy talked to her and attempted to keep her calm. The fire department responded and was able to remove the woman from underneath all the rubble.”
A woman alone in the home reported minor injuries, Southern Marin Fire Chief Chris Tubbs told the Marin Independent Journal. At least five cars were caught in the slide, which triggered the evacuation of about 50 homes in the area, the newspaper reported.
“This will take days before the debris is all cleaned up and things go back to normal,” Tubbs told the paper.
